Low noise amplifiers (LNAs) are used throughout wireless communications devices. One prominent use for LNAs is in the amplification of radio frequency (RF) receive signals so that these signals can be properly processed by downstream circuitry in a wireless communications device. Because a single LNA may amplify RF receive signals within different RF frequency bands, each of which may require a different amount of amplification so that they may be properly processed, there is often a desire for variable gain LNAs capable of providing multiple gain responses. FIG. 1 shows exemplary conventional LNA circuitry 10 capable of providing multiple gain responses.
The conventional LNA circuitry 10 includes an input node IN, an output node OUT, an amplifier 12, an input attenuator 14, an output attenuator 16, a first bypass switch SW_BP1, a second bypass switch SW_BP2, and control circuitry 18. The amplifier 12 is configured to provide a gain response between an amplifier input node A_IN and an amplifier output node A_OUT. Generally, this gain response provides the maximum desired gain of the conventional LNA circuitry 10. The input attenuator 14 is coupled between the input node IN and the amplifier input node A_IN and configured to provide a desired amount of attenuation between the input node IN and the amplifier input node A_IN in order to change a gain response of the conventional LNA circuitry 10. The output attenuator 16 is coupled between the amplifier output node A_OUT and the output node OUT and similarly configured to provide a desired amount of attenuation between the amplifier output node A_OUT and the output node OUT in order to change a gain response of the conventional LNA circuitry 10. The first bypass switch SW_BP1 is coupled between the amplifier input node A_IN and the amplifier 12, and the second bypass switch SW_BP2 is coupled in parallel with the amplifier 12 between the amplifier input node A_IN and the amplifier output node A_OUT. Together, the first bypass switch SW_BP1 and the second bypass switch SW_BP2 are configured to bypass the amplifier 12 when a zero gain response (i.e., 0 decibel (dB) gain response) or a negative gain response (i.e., negative dB gain response) is desired from the conventional LNA circuitry 10. The control circuitry 18 is configured to provide control signals to each one of the input attenuator 14, the output attenuator 16, and the bypass switch SW_BP in order to provide a desired gain response between the input node IN and the output node OUT of the conventional LNA circuitry 10. For example, to provide a first gain response, which is the maximum gain response, the input attenuator 14 and the output attenuator 16 are controlled to directly pass signals (with no attenuation), the first bypass switch SW_BP1 is closed, and the second bypass switch SW_BP2 is open. Accordingly, the amplifier 12 is placed in the signal path of the conventional LNA circuitry 10 and provides the maximum gain response. To provide a second gain response, which is the minimum gain response, the input attenuator 14 and the output attenuator 16 are controlled to provide a maximum level of attenuation, the first bypass switch SW_BP1 is opened to remove the input capacitance thereof from the signal path of the conventional LNA circuitry 10, and the second bypass switch SW_BP2 is closed to bypass the amplifier 12. Accordingly, the amplifier 12 is removed from the signal path of the conventional LNA circuitry 10 and the minimum gain response is provided.
The first bypass switch SW_BP1 provides some insertion loss (e.g., 0.1 dB to 0.4 dB, depending on the frequency of operation thereof) that directly contributes to an overall noise figure of the LNA circuitry 10, thereby setting a floor for the noise level thereof. Further, due to the fact that the first bypass switch SW_BP1 is located in the main signal path of the amplifier 12, the linearity of the conventional LNA circuitry 10 is reduced when providing the first gain response (i.e., the maximum gain response). This is due to inherent non-linearity associated with switching elements. While the linearity of the switch may be improved, such a high performance switching element will add significant cost to the circuitry, and the effect of the first bypass switch SW_BP1 cannot be eliminated altogether. Further, using the input attenuator 14 and the output attenuator 16 to reduce the gain of the conventional LNA circuitry 10 over a large range is generally inefficient, as the power used by the conventional LNA circuitry 10 does not reduce in proportion to the gain provided thereby. This is because the gain response of the amplifier 12 does not change, but rather is altered using the input attenuator 14 and the output attenuator 16 to produce a desired gain. The power consumption of the conventional LNA circuitry 10 is thus the same at any gain level.
Accordingly, there is a need for LNA circuitry that is capable of providing a variable gain response without degrading the performance thereof.